Optical cooling of single-walled carbon nanotubes as revealed by their anti-Stokes Ramanspectra

Abstract

Semiconducting single-walled carbon nanotubes resonantly excited by the interbandE22S electronic transitions (at 1064 nm) display for the two components of theradial Raman band—one associated with the isolated tubes and the otherassociated with the bundled tubes—an anti-Stokes/Stokes Raman intensity ratio(IaS/IS) which deviates oppositely from the predictions of the Maxwell–Boltzmann formula. Acooling and heating vibration process, evidenced by an enhancement and diminishment of(IaS/IS), appears in the isolated and bundled nanotubes, respectively. Here we confirm a coolingprocess, observed only for semiconducting nanotubes, which emerges from the relaxation of theE22S excited state by the electronic relaxation fromE22S toE11S that precedes the spontaneous luminescence emission atE11S. Metallic nanotubes do not exhibit luminescence and no cooling effectis observed. Both semiconducting and metallic nanotubes show forthe bundled component of the radial Raman band an enhancement of(IaS/IS) such as is frequently observed in a coherent anti-Stokes Raman scattering process.